The present application relates to a retardation compensation plate used for compensating retardation of, for example, a liquid crystal panel, and a retardation compensator, a liquid crystal display device and a projection-type image display device using the same.
There has been known a projector device, realizing a large picture by performing enlarged projection of an image onto a screen, as a projection-type image display device. In particular in recent years, so-called liquid crystal projector device, which projects beams emitted from a light source onto a screen after being modulated by a liquid crystal display device, has been disseminated. The liquid crystal display device displays images, based on a display mode corresponded to species of liquid crystal molecules used for the liquid crystal panel.
For example, a VA (vertically aligned) mode liquid crystal display device has widely been known. The VA mode liquid crystal display device includes liquid crystal molecules, sealed between a pair of substrates constituting the liquid crystal panel, showing negative dielectric anisotropy. In the VA-mode liquid crystal display device, the liquid crystal molecules, under no electric field applied thereto, align nearly perpendicular to the main surface of the substrate, so that beam can pass through the liquid crystal layer almost without altering its polarization plane. As a consequence, an excellent black state is obtained under no applied voltage, by disposing polarizing plates on the upper and lower sides of the substrate. On the contrary, under electric field applied thereto, the molecules align obliquely to the main surface of the substrate, and the resultant birefringence rotates the polarization plane of the incident light beam. The VA-mode liquid crystal display device has an advantage over a TN (Twisted Nematic) mode liquid crystal display device, in that it realizes higher contrast.
The VA-mode liquid crystal display device is configured to obtain a birefringent property by obliquely aligning the liquid crystal molecules under an electric field applied thereto. Therefore, the device preliminarily aligns the liquid crystal molecules, under no electric field applied thereto, at an extremely small angle of inclination (pre-tilt angle). Because the liquid crystal molecules under no electric field applied thereto are aligned as being slightly inclined in this way, rather than being completely perpendicular to the main surface of the substrates, the liquid crystal panel induces residual retardation. For this reason, the polarization plane of incident light from the vertical direction rotates slightly, consequently resulting in leakage of light from the polarizing plate and lowered contrast.
The VA mode also gives retardation to obliquely incident light in the absence of electric field. Therefore the device lowers the contrast, when the cone angle of incident light increases (F# is decreased) to obtain higher brightness (luminance).
On the other hand, there has been known a projection-type image display device, such as a three-plate-type liquid crystal projector having three liquid crystal panels corresponded to each color of RGB. The projection-type image display device generally adopts a prism-type polarization beam splitter (PBS) for polarization splitting. However, the prism-type polarization beam splitter suffers from large angular dependence, and results in lowering in contrast. Limitation of the cone angle aimed at ensuring the contrast results in disadvantage in terms of brightness (luminance).
Thus, it has been proposed that a λ/4 plate (quarter-wave plate) is disposed between the liquid crystal panel and the polarizing plate to correct lowering in contrast induced by the prism-type polarization beam splitter (see Japanese Patent Publication No. 3019813 (hereinafter referred to as “patent document 1”)).
In place of using the prism-type polarization beam splitter, it also has been proposed that a wire-grid polarizer showing only a small angular dependence, a reflection-type liquid crystal panel, and a retardation plate compensating the residual retardation ascribable to the pre-tilt angle of the liquid crystal panel and retardation to obliquely incident light are combined (see Japanese Unexamined Patent Application Publication No. 2005-18071 (hereinafter referred to as “patent document 2”)).
Japanese Unexamined Patent Application Publication No. 2006-189753 (hereinafter referred to as “patent document 3”) discloses a liquid crystal projector by using a retardation compensation element composed of a first optically anisotropic layer having high-refractive-index layers and low-refractive-index layers, made of an inorganic material, alternately stacked therein, and a second optically anisotropic layer having polymerizable liquid crystal compound kept in hybrid alignment, so that angle of alignment of molecules thereof is varied in the thickness-wise direction of the liquid crystal element, and thereby compensating retardation induced by the liquid crystal element.
However, rotation of the polarization plane caused due to the pre-tilt angle varies depending on the liquid crystal panel, so that the residual retardation to be compensated varies from panel to panel. Because of variation in the angle of placement of other optical components, it is difficult to stably adjust the contrast, only simply by disposing a quarter-wave plate between the liquid crystal panel and the polarizing plate.
On the other hand, one possible method of adjusting the contrast may be such as rotating the quarter-wave plate around an axis which stands vertically on the center of the liquid crystal panel. The quarter-wave plate, however, causes a large amount of variation in retardation with respect to the angle of rotation, and needs an extreme precision in rotation of the quarter-wave plate (0.5° or smaller, for example), making it difficult to optimize the contrast.
The combination of an inorganic optical multi-layered film and a liquid crystal described in the patent document 3 raises the following problems.
First of all, the liquid crystal largely varies the retardation in association with changes in temperature, proving only a limited environmental resistance. A high level of environmental resistance is required for the retardation compensation plate which is exposed to high-luminance beam in the projection-type image display device.
When a wire-grid polarizer, as an alternative of the prism-type polarization beam splitter, having only a small angular dependence, a reflection-type liquid crystal panel, and a retardation compensation plate compensating the residual retardation ascribable to the pre-tilt angle of the liquid crystal panel and retardation caused by obliquely incident light the liquid crystal panel or retardation are combined in the projection-type image display device such as three-plate-type liquid crystal projector, in-plane retardation required for the retardation compensation plate will be extremely small. Any effort of controlling such fine retardation will need a very precise technique of coating, raising difficulties in controlling the retardation and in-plane non-uniformity in the direction of optical axis.
Moreover, when liquid crystal is used for controlling such fine retardation, in-plane difference in refractive index (Δn0) generally has a value of as large as Δn0>0.1. If Δn0 has a large value, state of polarization of the reflected beam is partially converted due to difference in reflection coefficients between the normal wave and the abnormal wave, and thereby leakage of light from the wire-grid and lowering in contrast are observed. Because an AR coating for preventing reflective light can be optimized only for a single refractive index, it is very difficult to prevent reflection of both of normal wave and abnormal wave. Large Δn0 therefore degrades the anti-reflection effect, and thereby lowers the contrast. Although lowering in Δn0 may be an effective countermeasure, use of liquid crystal raises a need of complicated process such as immersion thereof into an isotropic compound, making it difficult to lower Δn0.